There are different things you might say in different contexts to imply that an action is short-lived and will be finished soon.
Ich esse nur schnell was.
Ich fahre mal kurz einkaufen.
Ich trainiere noch schnell zuende/fertig.
Ich wische eben durch.
Ich rauche gerade noch eine.
Ich mache bloß noch die Maschine an.
And more can probably be added by other users. (The third probably doesn't really fit here; it's more of an I'll just finish something; but they're closely related.)
While many are used interchangeably, not all can be switched. Ich esse eben was has a different meaning to it than ich esse nur schnell was.
Note that the suggestions you made, nur and einfach would be wrong in this context. Nur alone would be taken to mean only (ich esse nur was), and einfach would be something more like simply.
Also note that different versions are preferred over others in different parts of the German-speaking world. There are quite a few that would raise many eyebrows in certain areas for being uncommon.
The best suggestion I can give anybody wanting to learn rules is to shadow a German speaker and copy them.